SQUID elements are put in liquid helium, liquid nitrogen, and so on in a heat insulating container (cryostat etc.), and are maintained at a very low temperature. In a circuit using a direct current SQUID (dc-SQUID) element of a recent typical SQUID element, if magnetic flux is added to a SQUID element loop of the circuit using the SQUID element, which includes a Josephson junction, the voltage generating between terminals of the loop oscillates with a frequency depending on the strength of flux quanta at the Josephson junction. By detecting the oscillating voltage, magnetic flux intersecting the loop is detected with a high sensitivity.
To set the Josephson junction in a superconducting state, it is necessary to cool the Josephson junction under its superconduction temperature. As a structure to cool SQUID elements as mentioned above, the following structures are well known, that is: a structure in which SQUID elements are put and cooled in coolant such as liquid helium, liquid nitrogen, etc., in a cryostat for containing SQUID elements, or a structure in which SQUID elements are directly cooled by a refrigerator.
Further, as to the above cryostat for containing SQUID elements, the cryostat is placed along with a measured sample in a magnetic shield room surrounded by a shield member made of ferro-magnetic material such as permalloy in order to shield external magnetic flux. An example of a conventional cryostat for containing SQUID elements which are put and cooled in coolant such as liquid helium, liquid nitrogen, etc., is disclosed in Japanese Patent Application Laid-Open Hei. 7-321382.
In a conventional-type cryostat for containing SQUID elements which are put and cooled in coolant such as liquid helium, etc., since a group of SQUID elements is put in coolant, the troupe of SQUID elements are uniformly cooled at the same temperature. However, although the cryostat is composed by using a vacuum heat-insulating container because heat enters into a very low temperature part from the outside of a room temperature, the liquid helium gradually evaporates. Accordingly, it is necessary to supply coolant every week. Since the supply operations of liquid helium are very complicated, and liquid helium is very expensive, an increase of the operational cost is indispensable.
In a structure to solve the above-mentioned problems, for example, in the structure of an image processing apparatus for a medical measurement of nuclear magnetic resonance, using a superconducting magnet, which is disclosed in Japanese Patent Application Laid-Open Hei. 7-321382, a refrigerator is incorporated into a cryostat with a vacuum heat-insulating structure, and the heat entering into a very low temperature part from the outside of a room temperature is removed by the refrigerator in order to reduce the amount of vaporizing liquid helium. However, it is still necessary to supply liquid helium every a half of a year, and this supply operations of liquid helium are still very complicated. Moreover, if the vacuum in the vacuum heat-insulating structure is broken down, the liquid helium in the cryostat for containing SQUID elements instantaneously expands, and the pressure in the cryostat rapidly increases, which may break the cryostat. Thus, the safety is not completely secured by the above structure.
Further, when the cryostat for containing SQUID elements is used under an operational condition in which the cryostat is declined, or a measured sample is measured from a place lower than the measured sample in the upper direction by the SQUID elements in the cryostat which has been inverted, the head parts of the SQUID elements are exposed to a gas space by the declining surface of the coolant such as liquid helium, which causes insufficient cooling of the SQUID elements, or the coolant spills from the cryostat, which makes the measurement impossible.
Furthermore, a method solving the above-mentioned problem is disclosed on page 430 in Journal: Cryogenic Engineering, Vol. 28, No. 8 (1993). The SQUID elements in the cryostat for containing SQUID elements used in the method are cooled by a refrigerator. When the SQUID elements are cooled, an element supporting and cooling member made of solid material such as metal is cooled by the refrigerator, and the SQUID elements are cooled in a vacuum via the element supporting and cooling member.
However, since the SQUID elements are used to detect a very small quantity of magnetic flux, the element supporting and cooling member provided near the SQUID elements must be made of non-conductive material in order to prevent generation of eddy current due to the magnetic flux to be measured. Accordingly, the thermal conductivity of the non-conductive material is usually low. Moreover, the SQUID elements are detachably connected to the element supporting and cooling member by taking maintenance or exchange of the SQUID elements into consideration. Therefore, the SQUID elements are non-tightly attached to the element supporting and cooling member with screws, and the thermal conduction degree at the contact places between the member and each element cannot be uniformly controlled. Thus, temperature variance is caused among the cooled SQUID elements.
Furthermore, since the element supporting and cooling member and the SQUID elements are placed in a vacuum space, the member and the elements receive radiation heat from the parts of the structure surrounding the member and the elements, whose temperature is higher than that of the cooled member and elements. Further, since the quantity of the radiation heat transferred to the member and the element depends on the positional relation between the parts emitting radiation heat and the member or each element receiving the emitted radiation heat, a heat-receiving area in the member or each element, and so on, the variance is caused among quantities of heat received by the member and the elements. Accordingly, temperature variance is caused among the cooled SQUID elements. Since the sensitivity of a SQUID element is sensitive to its temperature, if there is temperature variance among the cooled SQUID elements, sensitivity variance is also caused among the cooled SQUID elements, which considerably degrades the accuracy in a measurement with the SQUID elements.
An object of the present invention is to provide a cryostat and a magnetism measurement apparatus using the cryostat which can secure the safety of a measured sample such as a human body even if a container in the cryostat should break down.
Another object of the present invention is to provide a cryostat and a magnetism measurement apparatus using the cryostat in which the evaporation amount of coolant to cool measurement elements such as SQUID elements is very small, and it is substantially unnecessary to supply the coolant.
Further, another object of the present invention is to provide a cryostat and a magnetism measurement apparatus using the cryostat in which cooled measurement elements such as SQUID elements can be declined toward a measured sample in all direction, that is, in the vertical, horizontal, or other declined directions.
Furthermore, another object of the present invention is to provide a cryostat and a magnetism measurement apparatus using the cryostat in which the required measurement accuracy is kept by uniformly cooling measurement elements such as SQUID elements.
Moreover, another object of the present invention is to provide a cryostat and a magnetism measurement apparatus using the cryostat in which the pressure in the cryostat does not increase beyond an the atmospheric pressure in a steady operation at a low temperature.
Additionally, another object of the present invention is to provide a cryostat and a magnetism measurement apparatus using the cryostat in which the evaporation amount of liquidized gas used for cooling coolant is reduced.